Dual-ratio master cylinder

ABSTRACT

The present invention is a dual-ratio master cylinder comprising a housing with coaxially disposed cylinders of different diameters which have pistons of matching diameters operable therein. The larger of the pistons has a bore therein of a diameter matching the diameter of the smaller of the two cylinders. An additional piston matching the diameter of the bore and the diameter of the small cylinder is disposed therein and urged into contact with a wall in the bore by means of a spring interposed between it and the small piston. The additional piston is provided with a fluid passage therethrough and a valve-type sensing means which provides fluid communication with the cylinders at a predetermined value of the valve-type sensing means.

United States Patent [1113,631,676

[72] Frederick A-Kl'usemark 3,478,518 11/1969 Lugerquist 60/54.6A2,448,194 8/1948 Schnell 60/5416A Nogzii i g 1970 PrimaryExaminer-Martin P. Schwadron [45] Patented Jan 4 1 972 AssistantExaminer-A. M. Zupcic [73] Assignee Borg- Valuer CorporationAttorneys-Donald W. Banner, William S. McCurry and John Chicago, 1]], W.Butcher [54] DUAL-RATIO MASTER CYLIND ABSTRACT: The present invention isa dual-ratio master 8 Claims,4Drawing Figs. Cylinder comprising ahousing with coaxially disposed cylinders of different diameters whichhave pistons of matching [52] U.S.Cl 60/54.6 A, diameters operabletherein The target of the pistons has a 60/54'6 bore therein of adiameter matching the diameter of the [S1] lnt.Cl FlSb 7/00 smaller ofthe two cylinders. An additional piston matching the diameter of thebore and the diameter of the small cylinder A is disposed therein andurged into contact with a wall in the bore by means of a springinterposed between it and the small [56] References cued piston. Theadditional piston is provided with a fluid passage [50] Field of SearchUNITED STATES PATENTS therethrough and a valve-type sensing means whichprovides 3,561,213 2/ 1971 Shiber 60/54.6 A fluid communication with thecylinders at a predetermined 1,21 2/197l Krusemark 60/54.6 A value ofthe valve-type sensing means. 2,343,900 3/1944 Groves 60/54.6 A2,343,901 3/1944 Groves 60/54.6 A

PATENIEU Jm 4:372 353L 7 sum 2 OF 2 N5 ED NW4 DUAL-RATIO MASTER CYLINDERSUMMARY OF THE INVENTION The present invention relates to a dual-ratiomaster cylinder and comprises a housing having a large cylinder and asmall cylinder, each having large and small pistons operable therein,respectively. A small second piston is carried in the small cylinder anda bore in the large piston which is urged against a wall in the bore byspring means interposed between the two small pistons. A hydraulicpressure sensing means carried by the small second piston iscommunicable with fluid passages which interconnect the cylinders with apassage in the small second piston.

Initially, in operation the fluid interposed between the small secondpiston and the wall of the bore in the large piston causes the twopistons to move in unison thereby delivering fluid in the braking systemby the large piston. This may be termed the first stage. When the fluidpressure in the small cylinder reaches a predetermined value, thepressure-sensing means opens fluid communication with the cylinderswhich initiates the second stage of fluid delivery to the system therebyreducing pedal effort on further movement of the pedal because fluidfrom the small cylinder provides pressure against the face area of theforward end of the large piston, thus in this stage the small and largepistons no longer move in unison; the small piston moving further thanthe large piston during the second stage of operation.

The modified form of the present invention has an added feature, namely,a rod passing through the large piston which displaces fluid in the borein the large piston approximately at the beginning of the second stage.

The purpose of the present invention is to provide an improveddual-ratio master cylinder which, during the initial stage of operation,provides a large volumetric supply of fluid to the system, which at apredetermined pressure, a second stage is initiated which reduces pedalefi'ort and has substantially no further pedal travel than theconventional system.

The present invention also provides a fail-safe means which causes areversion to the first stage in the event pedal travel is excessive;obviously a high volumetric supply of fluid is delivered to the system.

THE DRAWINGS FIG. 1 is a vertical sectional view of a dual-ratio mastercylinder according to the present invention, the rearward portion shownoffset;

FIG. 2 is a view corresponding to a portion of FIG. 1 showing adifferent type of seal;

FIG. 3 is a vertical sectional view of a modified form of a dual-ratiomaster cylinder shown in FIG. 1, the rearward end portion being offset;and I FIG. 4 is a longitudinal sectional view, partially in plan, of theoffset portion of FIG. 3 taken on line 44 of FIG. 3.

DESCRIPTION Referring now to the drawings, there is shown in FIG. 1 afluid reservoir 9, below which, and integral therewith, is a rela tivelysmall cylinder and a relatively large cylinder 11 in coaxial alignment.Relatively small first and second pistons 12 and 13 respectively areoperably disposed in said relatively small cylinder 10. The piston 13 isalso operably disposed in a bore 14 in a relatively large piston 15operably disposed in the relatively large cylinder 11. A protuberance 16extends from the relatively large piston 15. A spring 17 is interposedbetween the relatively small first and second pistons, 12 and 13respectively, thereby urging the relatively small second piston 13against a wall 18 at the end of the bore 14.

The relatively small second piston 13 is provided with a longitudinalpassage 19 extending therethrough and a check valve passage 20consisting of a small-diameter passage 21 and a larger-diameter passage22. This arrangement of components may be termed a check valve means.Disposed in the larger-diameter passage 22 is a ball 23 which is urgedagainst the small-diameter passage 21 by a spring 24 interposed betweenthe ball 23 and a closure 25. The larger-diameter passage 22 is providedwith a port 26 communicable with the relatively small and largecylinders 10 and 11 respectively.

The first relatively small piston 12 is provided with an O- ring 27 atits forward end and a lip seal 28 at its rearward end and the relativelysmall second piston 13 is provided with a lip seal 29 at its forward endand an O-ring 30 at its rearward end. A flat-faced seal 31 shown in FIG.2 is interposed between the rearward end of the relatively small secondpiston 13 and the wall 18 of the bore 14 which is modification of thatshown in FIG. 1. Lip seals 32 and 33 are provided at the forward andrearward ends respectively of the relatively large piston 15.

Recuperating ports 34 and 35 provide fluid communication between thereservoir 9 and the relatively small and relatively large cylinders 10and 11, respectively, in close proximity to the lip seals 28 and 33respectively. Reservoir ports 36 and 37 are in fluid communication withthe relatively small and relatively large cylinders 10 and 11respectively and are rearward of the lip seals 28 and 33 respectively.

A fail-safe protuberance 38 extends from the rearward end of therelatively small first piston 12 a predetermined distance from theforward end of the relatively small second piston 13 and serves as afail-safe means hereinafter more fully explained. An additionalrelatively large piston 39 having protuberances 16a and 16b at oppositeends thereof is shown slideably disposed in the relatively largecylinder 11 in a position rearward of the relatively large piston 15 andis provided with a pair of lip seals 40 and 41, each being attached toan end of the relatively large piston 39. The relatively large piston 39is provided with a stop means 45 whereby the rearward movement of therelatively large piston 39 is limited. A recuperating port 46 providesfluid communication between the fluid reservoir 9 and the relativelylarge cylinder 11 forward of the lip seal 41 and in close proximitythereto. A reservoir port 47 is communicable with the relatively largecylinder 11 rearward of the lip seal 41. A pair of fluid outlets 48 and49 are shown in communication with the relatively large cylinder 11.Each outlet is adapted to be connected to a hydraulic line leading to apair of wheel brakes.

A bleeder 50 communicable with the forward end of the relatively largecylinder 11 serves to remove trapped air.

OPERATION The operation of the embodiment shown in FIG. 1 is as follows.

The application of pressure against the relatively small first piston 12applies fluid pressure against the face of the relatively small secondpiston 13 which at its far end is in contact with the wall 18. Alongitudinal passage 19 extends through the relatively small secondpiston 13 which by means of the O-ring 30 seals off fluid communicationwith the relatively large cylinder 11 thus causing the relatively largepiston 15 to move in unison with the relatively small pistons 12 and 13thereby delivering a relatively large volumetric supply of fluid to ahydraulic line leading to the wheel cylinders.

When the hydraulic pressure in the system reaches a predetermined value,the ball 23 of the check value means provides fluid communication withthe relatively small and large cylinders 10 and 1 1 respectively thusinitiating the second stage of operation during which fluid from therelatively small cylinder 10 is delivered against the face area of therelatively large piston 15 which, of course, includes the wall 18.Hence, at this stage, the pedal effort is reduced because the relativelysmall first piston 12 travels a further distance than the relativelylarge piston 15 in the volumetric supply of fluid to the wheel brakes.

It should be noted that, although the foot pedal travels slightlyfarther during the second stage than the conventional system, this iscompensated for by the use of a larger piston than that used in theconventional system whereby the pedal travels a shorter distance ascompared with the conventional system and manifestly at a time when thebrake is taken up which requires but minimal effort.

A fail-safe protuberance 38 serving as a fail-safe means will contactthe forward face of the relatively small second piston 13, in the eventthat pedal travel becomes excessive, thereby causing the relativelysmall and large pistons to travel in unison and thus deliver the optimumin volumetric fluid supply.

The independent delivery of hydraulic fluid to each of a pair of wheelbrakes is provided by the use of the relatively large piston 39 whichhas a pair of lip seals 40 and 41 one each mounted on the end facesthereof. The areas of the relatively large cylinder 11 forward of thelip seals 40 and 41 are provided with fluid outlets 48 and 49,respectively, each adapted to be connected to one each of a hydraulicline leading to a pair of wheel brakes whereby, in operation, fluiddelivered from the master cylinder is delivered separately to one eachof a pair of wheel brakes.

A modified form of the invention identified in FIG. 1 is shown in FIG.3. The purpose of this modified form of the invention is to provide ameans whereby a larger volumetric supply of fluid may be provided duringthe second stage of operation. The use of this means necessitatesrearrangement and changes of some of the components. The componentswhich are identical to those shown in FIG. 1 except as to size of someare given the same identifying number.

Referring now to FIG. 3, a relatively large piston a, a modified form ofrelatively large piston 15, is provided with a bore 51 extending axiallythrough a wall 18a and having slideably operable therein a fluiddisplacement rod 52 provided with a seal 53 and an annular flange 54serving as a stop means against the wall 18a.

A relatively small piston 13a is provided with a bore 55 having a spring56 mounted therein and in contact with the fluid displacement rod 52thus urging the annular flange 54 against the wall 180. Longitudinalpassage 19a, is in communication with the bore 55 and the relativelysmall cylinder 10. A wall 57 at the rearward end of the relatively largecylinder 11 serves as a stop means for the fluid displacement rod, 52. Areturn spring 58 is interposed between the wall 57 and the relativelylarge piston 15a.

FIG. 4 shows a pair of laterally disposed cylinders 59 and 60longitudinally disposed below a fluid reservoir 90 and having slideablydisposed therein a pair of pistons 61 and 6 2 respectively provided witha pair of lip seals 40a and 411.: respectively at their forward ends.Reservoir ports 63 and 64 are in fluid communication with the cylinders59 and 60 respectively having disposed therein tipping valves 65 and 66respectively whereby fluid communication is provided to outlets 67 and68 respectively one each of which is adapted to be connected to one pairof hydraulic lines leading to a pair of wheel brakes. A pair of returnsprings 69 and 70 are interposed between the pistons 61 and 62respectively and a pair of walls 71 and 72 respectively whereby thepistons 61 and 62 are urged against the wall 57.

The operation of the embodiment shown in FIGS. 3 and 4 is similar inperformance to the embodiment which is shown in FIG.- 1 except for anadditional feature, namely the fluid displacement rod 52, which moves inunison with the relatively large piston 16a until it contacts the wall57 which serves as a stop means. At this stage, the face area of therelatively large piston lSa is reduced to the extent of the face area ofthe fluid displacement rod 52. Thus, during the second stage ofoperation, the pedal effort will be less than the embodiment shown inFIG. 1, and still have substantially the same pedal travel.

An additional modification shown in FIG. 4 pertains to a means forindependently supplying fluid to wheel brakes, which comprises the pairof laterally disposed cylinders 59 and 60 longitudinally disposed belowa fluid reservoir 90 provided with a pair of pistons 61 and 62respectively which independently are connected to hydraulic lines oneeach of which leads to a pair of wheel brakes.

This modification was necessary in order to provide a stop means, namelythe wall 57, for the fluid displacement rod S2.

Various of the features of the invention have been particu' larly shownand described, it should be obvious to one skilled in the art thatvarious modifications may be made therein without departing from thescope of the invention.

I claim:

1. A pressure cylinder for a hydraulic brake system including a housing;

A. a first relatively small cylinder at the forward end of said housing;

8. a relatively large cylinder in coaxial alignment with said firstrelatively small cylinder;

C. a relatively large first piston having a bore therein operablydisposed in said relatively large cylinder;

D. a relatively small first piston operably disposed in said relativelysmall cylinder and a relatively small second piston having alongitudinal passage axially extending therethrough, operably disposedin the said relatively small cylinder and in a bore in said relativelylarge piston;

E. a spring interposed between the said first and second relativelysmall pistons whereby the said relatively small second piston is urgedinto contact with a wall of said bore in said relatively large piston;

F. a sealing means disposed in the area of the end of the outerperiphery of said relatively small second piston;

G. a pressure-sensing means disposed in the said relatively small secondpiston between small-diameter and largerdiameter passages which arecommunicable with the said longitudinal passage and the area in therelatively large cylinder forwardly of the said relatively large piston;and

H. a fluid reservoir in said housing provided with passages communicablewith said relatively small cylinder and relatively large cylinder.

2. A pressure cylinder for a hydraulic brake system including a housing;

A. a relatively small cylinder at the forward end of said housing;

B. a relatively large cylinder in coaxial alignment with said relativelysmall cylinder;

C. a relatively large first piston having a bore therein operablydisposed in said relatively large cylinder;

D. a relatively small first piston operably disposed in said relativelysmall cylinder;

E. a relatively small second piston operably disposed in said relativelysmall cylinder and in a bore in said relatively large piston;

F. a spring interposed between the said first and second relativelysmall pistons whereby the said relatively small second piston is urgedinto contact with a wall of said bore in said relatively large piston;

G. a sealing means disposed in the area of the end of the outerperiphery of the said relatively small second piston;

H. a pressure-sensing means disposed in the said relatively small secondpiston between small-diameter and largerdiameter passages which arecommunicable with the said relatively small cylinder and relativelylarge cylinder in the area of the said relatively large piston;

I. a fluid reservoir in said housing provided with passages communicablewith said relatively small cylinder and relatively large cylinder;

J. a first fluid reservoir recuperating port forward of the saidrelatively small first piston;

K. a second fluid reservoir recuperating port forward of the saidrelatively large piston;

L. a first fluid reservoir port rearward of the forward end of the saidrelatively small first piston;

M. a second fluid reservoir port rearward of the forward end of the saidrelatively large piston;

N. a first-piston lip seal at the forward end of the said relativelysmall first piston;

O. a second piston lip seal at the forward end of the said relativelylarge piston;

P. a third piston lip seal at the rearward end of the said relativelylarge piston;

Q. a fourth piston lip sea! at the rearward end of the said relativelysmall second piston; and

R. a fluid outlet interposed between the said relatively large cylinderand the said hydraulic system.

3. A pressure cylinder as in claim 2 wherein the said sealing means isan O-ring.

4. A pressure cylinder as in claim 2 wherein the said sealing means isan annular ring disposed between the end of the outer periphery of saidrelatively small second piston and the said wall of the said bore.

5. A pressure cylinder as in claim 2 wherein a bleeder is provided atthe forward end of the relatively large cylinder.

6. A pressure cylinder as in claim 3 wherein the said relatively smallfirst piston is provided with a fail-safe protuberance engageable withsaid relatively small second piston thereby serving as a fail-safe meanswhich causes a reversion to first stage operation.

7. A pressure cylinder for a hydraulic brake system including a housing;

A. a relatively small cylinder at the forward end of the said housing;

B. a relatively large cylinder in coaxial alignment with said relativelysmall cylinder;

C. first and second relatively small pistons operably disposed in saidrelatively small cylinder, said relatively small second piston extendinginto a bore in a first relatively large piston having a firstprotuberance;

D. a longitudinal passage extending through said second relatively smallpiston;

E. a spring interposed between the said first and second relativelysmall pistons whereby the said relatively small second piston is urgedinto contact with the wall of said bore in said first relatively largepiston;

F. a fail-safe protuberance extending from said relatively small firstpiston;

G. a sealing means disposed in the area of the end of the outerperiphery of the said second relatively small piston;

H. a pressure-sensing means disposed in the said relatively small secondpiston between small-diameter and largediameter passages which arecommunicable with the said longitudinal passage and the area in thelarge cylinder forwardly of the said first relatively large piston;

l. a second relatively large piston provided with second and thirdprotuberances extending in opposite directions toward the said firstprotuberance and a wall at the end of the said large cylinder,respectively;

J. a second spring interposed between the said first relatively largepiston and the forward end of the said second relatively large pistonthereby urging the said second relatively large piston in a directiontoward a stop means disposed in a reduced diameter between opposite endsof the said second relatively large piston;

K. a third spring interposed between the rearward end of the said secondrelatively large piston and the said wall at the end of the saidrelatively large cylinder;

L. a fluid reservoir in said housing provided with first, second, andthird recuperating ports one each of which is rearwardly of the saidfirst relatively small piston, rearwardly of the said first relativelylarge piston and rearwardly of the said second relatively large piston,respectively;

M. a first fluid outlet forwardly of the said second relatively largepiston communicable with brake means for a first pair of wheels; and

N. a second fluid outlet rearwardly of said second relatively largepiston communicable with brake means for a second pair of wheels.

8. A pressure cylinder for a hydraulic brake system including a housinghaving relatively small and large cylinders disposed therein;

A. said relatively small cylinder positioned forwardly of saidrelatively large cylinder;

B. a relatively large first piston operably disposed in said relativelylarge cylinder;

C. a relatively small first piston in coaxial alignment operablydisposed in said relatively small and large cylinders respectively; D. arelatively small second piston having a longitudinal passage axiallyextending therethrough operably disposed in said relatively smallcylinder and a bore in said relatively large piston;

E. a spring interposed between said relatively small first and secondpistons whereby the relatively small second piston is urged into contactwith a wall of the said bore in said relatively large piston.

F. a sealing means disposed in the area of the end of the outerperiphery of the said relatively small second piston;

G. a pressure-sensing means disposed in the said relatively small secondpiston between small-diameter and largediameter passages respectivelywhich are communicable with the said longitudinal passage and therelatively large cylinder area forward of the said relatively largepiston; and

H. a fluid reservoir in said housing provided with passages communicablewith said relatively small and large cylinders.

UNITED STATES PATENT @FFICE QERTHFIQATE @F fiQRREQNQN Patent No. 3: 3Dated January 4, 1972 Inventor) Frederick A. Krusemark It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column 5, line 37, "large" should read larger Column 6, line 36, cancelthe period after ."piston" and insert a semi-colon Signed and sealedthis 17th day of October 1972.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOII'SCHALK Attesting Officer Commissionerof Patents USCOMM-DC 60878-P69 V ".5. GOVIRNMENT PRIN'HNG OFFICE: DIDO-3al-S-.

1. A pressure cylinder for a hydraulic brake system including a housing;A. a first relatively small cylinder at the forward end of said housing;B. a relatively large cylinder in coaxial alignment with said firstrelatively small cylinder; C. a relatively large first piston having abore therein operably disposed in said relatively large cylinder; D. arelatively small first piston operably disposed in said relatively smallcylinder and a relatively small second piston having a longitudinalpassage axially extending therethrough, operably disposed in the saidrelatively small cylinder and in a bore in said relatively large piston;E. a spring interposed between the said first and second relativelysmall pistons whereby the said relatively small second piston is urgedinto contact with a wall of said bore in said relatively large piston;F. a sealing means disposed in the area of the end of the outerperiphery of said relatively small second piston; G. a pressure-sensingmeans disposed in the said relatively small second piston betweensmall-diameter and larger-diameter passages which are communicable withthe said longitudinal passage and the area in the relatively largecylinder forwardly of the said relatively large piston; and H. a fluidreservoir in said housing provided with passages communicable with saidrelatively small cylinder and relatively large cylinder.
 2. A pressurecylinder for a hydraulic brake system including a housing; A. arelatively small cylinder at the forward end of said housing; B. arelatively large cylinder in coaxial alignment with said relativelysmall cylinder; C. a relatively large first piston having a bore thereinoperably disposed in said relatively large cylinder; D. a relativelysmall first piston operably disposed in said relatively small cylinder;E. a relatively small second piston operably disposEd in said relativelysmall cylinder and in a bore in said relatively large piston; F. aspring interposed between the said first and second relatively smallpistons whereby the said relatively small second piston is urged intocontact with a wall of said bore in said relatively large piston; G. asealing means disposed in the area of the end of the outer periphery ofthe said relatively small second piston; H. a pressure-sensing meansdisposed in the said relatively small second piston betweensmall-diameter and larger-diameter passages which are communicable withthe said relatively small cylinder and relatively large cylinder in thearea of the said relatively large piston; I. a fluid reservoir in saidhousing provided with passages communicable with said relatively smallcylinder and relatively large cylinder; J. a first fluid reservoirrecuperating port forward of the said relatively small first piston; K.a second fluid reservoir recuperating port forward of the saidrelatively large piston; L. a first fluid reservoir port rearward of theforward end of the said relatively small first piston; M. a second fluidreservoir port rearward of the forward end of the said relatively largepiston; N. a first-piston lip seal at the forward end of the saidrelatively small first piston; O. a second piston lip seal at theforward end of the said relatively large piston; P. a third piston lipseal at the rearward end of the said relatively large piston; Q. afourth piston lip seal at the rearward end of the said relatively smallsecond piston; and R. a fluid outlet interposed between the saidrelatively large cylinder and the said hydraulic system.
 3. A pressurecylinder as in claim 2 wherein the said sealing means is an O-ring.
 4. Apressure cylinder as in claim 2 wherein the said sealing means is anannular ring disposed between the end of the outer periphery of saidrelatively small second piston and the said wall of the said bore.
 5. Apressure cylinder as in claim 2 wherein a bleeder is provided at theforward end of the relatively large cylinder.
 6. A pressure cylinder asin claim 3 wherein the said relatively small first piston is providedwith a fail-safe protuberance engageable with said relatively smallsecond piston thereby serving as a fail-safe means which causes areversion to first stage operation.
 7. A pressure cylinder for ahydraulic brake system including a housing; A. a relatively smallcylinder at the forward end of the said housing; B. a relatively largecylinder in coaxial alignment with said relatively small cylinder; C.first and second relatively small pistons operably disposed in saidrelatively small cylinder, said relatively small second piston extendinginto a bore in a first relatively large piston having a firstprotuberance; D. a longitudinal passage extending through said secondrelatively small piston; E. a spring interposed between the said firstand second relatively small pistons whereby the said relatively smallsecond piston is urged into contact with the wall of said bore in saidfirst relatively large piston; F. a fail-safe protuberance extendingfrom said relatively small first piston; G. a sealing means disposed inthe area of the end of the outer periphery of the said second relativelysmall piston; H. a pressure-sensing means disposed in the saidrelatively small second piston between small-diameter and large-diameterpassages which are communicable with the said longitudinal passage andthe area in the large cylinder forwardly of the said first relativelylarge piston; I. a second relatively large piston provided with secondand third protuberances extending in opposite directions toward the saidfirst protruburance and a wall at the end of the said large cylinder,respectively; J. a second spring interposed between the said firstrelatively large piston and thE forward end of the said secondrelatively large piston thereby urging the said second relatively largepiston in a direction toward a stop means disposed in a reduced diameterbetween opposite ends of the said second relatively large piston; K. athird spring interposed between the rearward end of the said secondrelatively large piston and the said wall at the end of the saidrelatively large cylinder; L. a fluid reservoir in said housing providedwith first, second, and third recuperating ports one each of which isrearwardly of the said first relatively small piston, rearwardly of thesaid first relatively large piston and rearwardly of the said secondrelatively large piston, respectively; M. a first fluid outlet forwardlyof the said second relatively large piston communicable with brake meansfor a first pair of wheels; and N. a second fluid outlet rearwardly ofsaid second relatively large piston communicable with brake means for asecond pair of wheels.
 8. A pressure cylinder for a hydraulic brakesystem including a housing having relatively small and large cylindersdisposed therein; A. said relatively small cylinder positioned forwardlyof said relatively large cylinder; B. a relatively large first pistonoperably disposed in said relatively large cylinder; C. a relativelysmall first piston in coaxial alignment operably disposed in saidrelatively small and large cylinders respectively; D. a relatively smallsecond piston having a longitudinal passage axially extendingtherethrough operably disposed in said relatively small cylinder and abore in said relatively large piston; E. a spring interposed betweensaid relatively small first and second pistons whereby the relativelysmall second piston is urged into contact with a wall of the said borein said relatively large piston. F. a sealing means disposed in the areaof the end of the outer periphery of the said relatively small secondpiston; G. a pressure-sensing means disposed in the said relativelysmall second piston between small-diameter and larger-diameter passagesrespectively which are communicable with the said longitudinal passageand the relatively large cylinder area forward of the said relativelylarge piston; and H. a fluid reservoir in said housing provided withpassages communicable with said relatively small and large cylinders.